1. Field of the Invention
This invention relates to an improvement applicable to electrical discharge machining equipment. More specifically, this invention relates to an improvement applicable to means for sensing the voltage across the gap in which electrical discharges occur for electrical discharge machining.
2. Description of the Related Art
It is required to maintain the geometrical length of the gap across which electrical discharges occur for electrical discharge machining at a constant value, for the purpose of maintaining a stable intensity for an electric current flowing in the gap for electrical discharge machining. To satisfy this requirement, it is essential to employ means for sensing the voltage across the gap in which electrical discharges occur for electrical discharge machining (Hereinafter referred to as the electric discharge machining gap voltage sensing means).
Presented below will be a description of electrical discharge machining equipment described paying particular attention to electric discharge machining gap voltage sensing means.
Referring to FIG. 1, a machining electrode 1 can be a rod electrode (which is employed in electrical discharge machining equipment for producing a recess), as shown in the drawing, or a wire electrode (not shown but employable in wire electrode type electrical discharge machining). A gap 12 in which electric discharges occur (hereinafter referred to as a machining gap) is maintained between the machining electrode 1 and a workpiece 2 to allow electric discharges to occur therein. Means 3 for generating a high frequency pulse wave electric voltage is allowed to select variable wave forms. An instrument current transformer 4 or the like is designed to sense the variation rate of electric current. In the equipment illustrated in FIG. 1, the transformer 4 is actually employed as a means for sensing an inductance fall component. An electric discharge machining gap voltage is sensed at a point 5, at which point the circuit of the high frequency pulse wave electric voltage generating means 3 is connected to the rod machining electrode 1, in the case of electrical discharge machining for producing a recess in the workpiece 2. On the other hand, an electric discharge machining gap voltage is sensed by means of electric feeder means or the like (not shown), in the case of wire electrode type electrical discharge machining, in which a wire electrode is employed.
An electric discharge machining gap voltage sensed at any of the points described above contains not only the genuine value of the electric discharge machining gap voltage but also error voltage components. The error voltage components include an inductance fall component and a resistance fall component which occur between the machining electrode 1 and the electric discharge machining gap voltage sensng point 5. The error voltage components also include an inductance fall component and a resistance fall component which occur between the workpiece 2 and the other electric discharge machining gap voltage sensing point (which is not shown in the drawing, but is one terminal of the instrument current transformer 4 or the ground point in this example). Particularly in the case of wire electrode type electrical discharge machining equipment, the inductance fall component is so large that the inductance fall component cannot be ignored. Therefore, it is required to sense the error voltage components and to subtract the voltage amount from the voltage between the two points at which an electric discharge machining gap voltage is sensed (between one end of the machining electrode 1 and the ground point in this embodiment), for the purpose of correcting the apparent electric discharge machining gap voltage sensed in the manner described above and to obtain the correct or true electric discharge machining gap voltage which is actually applied across the electrical discharge machining gap.
Means for removing the influence of the aforementioned error voltage components can be realized by employing a principle that the inductance fall component accounting for the major portion of the error voltage components is proportional to the variation rate of an electric current. Therefore, electrical discharge machining equipment available in the prior art is provided which electric discharge machining gap voltage correcting means 6 which functions to subtract the inductance fall component, which is determined following the output signal of the electric current variation rate sensing means 4 such as an instrument current transformer or the like, from the apparent electric discharge machining gap voltage (the voltage difference between the potential at one end of the machining electrode 1 and the ground potential, in this example) sensed in the aforementioned manner. The electric discharge machining gap voltage correcting means 6 is inherently an adder. The apparent electric discharge machining gap voltage (the voltage difference between the potential at one end of the machining electrode 1 and the ground potential, in this example) sensed in the aforementioned manner is applied to a first element 61. The output signal of the electric current variation rate sensing means 4 is applied to a second element 62 of the correcting means 6. Since the output signal of the electric current variation rate sensing means 4 is a negative value, the electric discharge machining gap voltage correcting means 6 actually performs subtraction.
Adjustment of the second element 62 is readily possible by adjusting the output signal of the electric discharge machining gap voltage correcting means 6 to 0 (zero). This is accomplished by short-circuiting the machining gap 12 and applying a high frequency pulse voltage between the point 5 and one end of the instrument current transformer 4.
Although the electric discharge machining gap voltage correcting means 6 is remarkably effective to remove the harmful effects of the inductance fall component, the harmful effects of the resistance fall component can not be ignored, particularly in the case where a thin wire electrode or a tungsten wire electrode is employed.